feat(notification): add target_mode enum constraint and merge register-notifications script

- Add NotificationTargetMode enum (new_users/exist_users/all_users/user_ids)
- Create Alembic migrations: drop duplicate indexes, add target_mode column
- Merge register-notifications.sh into dev-migrate.sh sync-notifications subcommand
- Shorten notification config path: static/notification/notifications -> static/notifications
- Update registration flow to dispatch notifications by target_mode
- Add is_first_registration to RegisterBonusResult for first-time user detection
- Remove dead code: link_published_notifications_to_user
- Update welcome_points.yaml to target new_users only
- Add 44 unit tests + 1 integration test, all passing

backend/tests/unit/test_notification_target_mode.py

@@ -0,0 +1,150 @@
+from __future__ import annotations
+
+from datetime import datetime
+from uuid import UUID, uuid4
+
+import pytest
+
+from schemas.enums import NotificationTargetMode
+from v1.notifications.service import NotificationService
+
+
+class _FakeNotification:
+    def __init__(
+        self,
+        *,
+        id: UUID,
+        target_mode: NotificationTargetMode = NotificationTargetMode.ALL_USERS,
+        status: str = "published",
+        deleted_at: datetime | None = None,
+    ):
+        self.id = id
+        self.target_mode = target_mode
+        self.status = status
+        self.deleted_at = deleted_at
+
+
+class _TrackingNotificationRepository:
+    def __init__(self, notifications: list[_FakeNotification]) -> None:
+        self._notifications = notifications
+        self.linked_notification_ids: list[list[UUID]] = []
+        self.linked_is_first: list[bool] = []
+
+    async def link_notifications_for_registered_user(
+        self, *, user_id: UUID, is_first_registration: bool
+    ) -> int:
+        target_modes: list[NotificationTargetMode]
+        if is_first_registration:
+            target_modes = [
+                NotificationTargetMode.NEW_USERS,
+                NotificationTargetMode.ALL_USERS,
+            ]
+        else:
+            target_modes = [NotificationTargetMode.ALL_USERS]
+
+        matched = [
+            n
+            for n in self._notifications
+            if n.status == "published"
+            and n.deleted_at is None
+            and n.target_mode in target_modes
+        ]
+        self.linked_notification_ids.append([n.id for n in matched])
+        self.linked_is_first.append(is_first_registration)
+        return len(matched)
+
+
+@pytest.fixture
+def notification_new_users() -> _FakeNotification:
+    return _FakeNotification(id=uuid4(), target_mode=NotificationTargetMode.NEW_USERS)
+
+
+@pytest.fixture
+def notification_all_users() -> _FakeNotification:
+    return _FakeNotification(id=uuid4(), target_mode=NotificationTargetMode.ALL_USERS)
+
+
+@pytest.fixture
+def notification_exist_users() -> _FakeNotification:
+    return _FakeNotification(id=uuid4(), target_mode=NotificationTargetMode.EXIST_USERS)
+
+
+class TestLinkNotificationsForRegisteredUser:
+    @pytest.mark.asyncio
+    async def test_first_registration_gets_new_users_and_all_users(
+        self,
+        notification_new_users: _FakeNotification,
+        notification_all_users: _FakeNotification,
+        notification_exist_users: _FakeNotification,
+    ) -> None:
+        repo = _TrackingNotificationRepository(
+            [notification_new_users, notification_all_users, notification_exist_users]
+        )
+        service = NotificationService(repository=repo)  # type: ignore[arg-type]
+
+        count = await service.link_notifications_for_registered_user(
+            user_id=uuid4(), is_first_registration=True
+        )
+
+        assert count == 2
+        linked_ids = repo.linked_notification_ids[0]
+        assert notification_new_users.id in linked_ids
+        assert notification_all_users.id in linked_ids
+        assert notification_exist_users.id not in linked_ids
+
+    @pytest.mark.asyncio
+    async def test_reregistered_user_only_gets_all_users(
+        self,
+        notification_new_users: _FakeNotification,
+        notification_all_users: _FakeNotification,
+        notification_exist_users: _FakeNotification,
+    ) -> None:
+        repo = _TrackingNotificationRepository(
+            [notification_new_users, notification_all_users, notification_exist_users]
+        )
+        service = NotificationService(repository=repo)  # type: ignore[arg-type]
+
+        count = await service.link_notifications_for_registered_user(
+            user_id=uuid4(), is_first_registration=False
+        )
+
+        assert count == 1
+        linked_ids = repo.linked_notification_ids[0]
+        assert notification_new_users.id not in linked_ids
+        assert notification_all_users.id in linked_ids
+        assert notification_exist_users.id not in linked_ids
+
+    @pytest.mark.asyncio
+    async def test_no_published_notifications_returns_zero(self) -> None:
+        repo = _TrackingNotificationRepository([])
+        service = NotificationService(repository=repo)  # type: ignore[arg-type]
+
+        count = await service.link_notifications_for_registered_user(
+            user_id=uuid4(), is_first_registration=True
+        )
+
+        assert count == 0
+
+    @pytest.mark.asyncio
+    async def test_only_new_users_notification_first_registration(self) -> None:
+        n = _FakeNotification(id=uuid4(), target_mode=NotificationTargetMode.NEW_USERS)
+        repo = _TrackingNotificationRepository([n])
+        service = NotificationService(repository=repo)  # type: ignore[arg-type]
+
+        count = await service.link_notifications_for_registered_user(
+            user_id=uuid4(), is_first_registration=True
+        )
+
+        assert count == 1
+
+    @pytest.mark.asyncio
+    async def test_only_new_users_notification_reregistered(self) -> None:
+        n = _FakeNotification(id=uuid4(), target_mode=NotificationTargetMode.NEW_USERS)
+        repo = _TrackingNotificationRepository([n])
+        service = NotificationService(repository=repo)  # type: ignore[arg-type]
+
+        count = await service.link_notifications_for_registered_user(
+            user_id=uuid4(), is_first_registration=False
+        )
+
+        assert count == 0